Hybrid foil-magnetic bearing

ABSTRACT

A method for bearing a rotor in which load is shared between a foil bearing part and a magnetic bearing part without using a calculated rotor trajectory corresponding to various rotor spin speeds. Power input to the magnetic bearing part is controlled in response to feedback of rotor position. In order to share the load on the fly so that the load is shared quickly as the rotor speed is changed, the power input is modified in response to rotor spin speed to reduce load capacity of the magnetic bearing part as rotor spin speed is increased and increase load capacity of the magnetic bearing part as rotor spin speed is decreased. Alternative methods of modifying power input include modifying integral gain and inputting rotor spin speed to an adaptive filter to continually reset the adaptive filter at the rotor spin speed. For use of the process with a thrust bearing, the magnetic and foil bearing parts are disposed on opposite sides of the thrust runner.

The present application claims priority of U.S. provisional patentapplication Ser. No. 60/059,005, filed Sep. 15, 1997, which applicationis hereby incorporated herein by reference.

The present invention relates generally to bearings.

Foil bearings, such as disclosed in U.S. patent application Ser. Nos.08/827,203 and 08/827,202, assigned to the assignee of the presentinvention, and U.S. Pat. Nos. 4,262,975; 4,277,113; 4,300,806;4,296,976; 4,277,112; and 4,277,111 of Hooshang Heshmat (either as soleor as joint inventor), an inventor of the present invention, whichapplications and patents are incorporated herein by reference, include asheet positioned to face a shaft portion for relative movement therebetween and means in the form of a corrugated shape having a pluralityof ridges or other suitable form for resiliently supporting the sheetthereby defining a compliant hydrodynamic fluid film bearing. Thebearing may be a journal bearing in which case the sheet is insurrounding relation to a shaft for relative rotational movement therebetween or a thrust bearing in which case the sheet bears a rotatingshaft runner. Stiffness and damping are provided in a foil bearing bythe smooth top foil or sheet and structural support elements which aresuitably designed to provide a compliant spring support of the desiredstiffness (or stiffness which is variable with load) and damping and bythe hydrodynamic effects of a gas film between the shaft and the smoothtop foil.

Magnetic bearings, such as disclosed in U.S. patent application Ser. No.09/046,334, which is assigned to the assignee of the present invention,and in U.S. Pat. Nos. 5,084,643; 5,133,527; 5,202,824; and 5,666,014 ofHsaing Ming Chen (either as sole or as joint inventor), an inventor ofthe present invention, which applications and patents are incorporatedherein by reference, include magnet means on a housing whichmagnetically interact with a shaft portion for adjusting the positionthereof during rotation thereof. A magnetic bearing may be provided aseither a journal or a thrust bearing.

Magnetic bearings may be classified as using either repulsive orattractive forces. Repulsive force systems often use permanent magnetswhile attractive force systems usually use electromagnets. Attractionelectromagnets are usually used for magnetic suspension systems(bearings) since stiffness nearly comparable to rolling element bearingscan be achieved and since active control permits variation of parametersas dictated by rotor system dynamics. An actively controlled magneticbearing generally comprises a stator which is wound with coils to createthe magnetic field and ferromagnetic laminations mounted on the rotor tointeract with the stator magnetic field.

Position sensors provide feedback for control of magnetic bearings. Biascurrents are conventionally applied to the electromagnets to supportstatic loads and set up an operating flux field for linearized control.Since the flux field is equivalent to a negative spring, the bearing isinherently unstable. For reliable rotor control, both rotor position andits rate of change need to be corrected. In other words, the activemagnetic bearing needs damping or velocity control, which is achieved byadding rotor velocity feedback to the current control. The rotorvelocity is generally estimated from the displacement measurementsthrough the use of a differentiator, phase-lead circuit, or stateobserver. In addition to dynamic stiffness and damping, basic rotorposition error feedback is required to statically center the rotor. Atypical magnetic bearing control is thus a gain and phase compensationnetwork which provides a summation of (1) the time-varying positionsignal for dynamic stiffness control (which may be called“Proportional”), (2) the integral of the position signal error forstatic stiffness control (which may be called “Integral”), and (3) thederivative of the time-varying position signal for damping (which may becalled “Derivative”). High static stiffness is provided to keep therotor centered in the bearing. With independent control of each of theseelements, the controller, which may resultingly be called a “PIDcontroller”, allows the magnetic bearing characteristics to be varied asa function of machine operation. Lead-lag or notch filter circuits areadded to the PID circuit to allow gain and phase compensation atresonant frequencies not covered by the PID circuit. Common rotordynamiccontrols include varying the bearing stiffness to alter lateralvibration modes, inserting damping to reduce dynamic motion, andgenerating rotating bearing forces to oppose or cancel rotor unbalanceand harmonic forces.

The mechanical simplicity of foil bearings makes them suitable forhigh-speed machines such as those with cryogenic turbo-rotors with bothexpander and compressor wheels running at tens of thousands rpm.However, a significant effort is required to design a set of foilbearings for any new application. Furthermore, foil bearings do not liftoff at low speed, thus requiring a coating on the foil for protectionthereof at low speeds during start-ups and shut-downs. To make along-lasting coating, uniform foil surface compliancy must be providedby design. Moreover, it is not easy to design an adequate amount ofcoulomb damping in the foil bearing crucial for rotor stability at highspeeds.

Active magnetic bearings are considered to be well suited for low speedoperations due to there being no metal contact, dynamic softness, andelectronically maneuverable stiffness and damping. However, activemagnetic bearings are vulnerable to rotor bending or structuralresonances, due to non-collocation of sensors and actuators (not in samelocation axially as bearing center), which can easily saturate poweramplifiers and make the control system unstable. Furthermore, it isdifficult to provide reliable and long-lasting back-up bearings foractive magnetic bearings. Conventional rolling-element type back-upbearings tend to have skidding wear and last for only a few rotor dropsdue to electric failures. Moreover, violent backward whirl may occur torender a rotor-bearing failure a disaster. However, some progress isbeing made to provide improved back-up bearings.

Since the foil bearing is considered to be advantageous for high speedoperation and the magnetic bearing for low-speed operation, it isconsidered advantageous to combine them into a hybrid bearing having theadvantages of each. The hybrid journal bearing is considered to provide,for some applications such as aircraft gas turbines, the followingbenefits. Since the specific capacity of a foil bearing is typicallyabout 500 lbs. per lb. of bearing weight and since that of the activemagnetic bearing is typically about 40 lbs. per lb. of bearing weight,the hybrid bearing should be much smaller, lighter in weight, andconsume less power than a pure active magnetic bearing, for the sameload capacity. The rotor may coast down safely on the foil bearing partin case of electric power loss to the magnetic bearing part. The foilbearing coating wear problem is no longer a problem because the magneticbearing part can take the load at low speeds. Sub-synchronous stabilitycan be enhanced by electronically generated damping of the magneticbearing part. Sub-synchronous stability may be enhanced byelectronically tuning the controller transfer function to obtaindesirable system dynamics. The tuning of the magnetic bearing part inhigh frequency range may be simplified because the rotor is supported bythe foil bearing part with Coulomb damping. The ability to independentlyvary bearing characteristics, provided by the active magnetic bearing,offers versatile rotor control. With the employment of an activemagnetic bearing part, rotor speed has no direct effect of the loadcapacity.

It is therefore considered desirable to provide a suitably controllablehybrid journal bearing. However, there is an eccentricity of the foilbearing part, as seen in FIGS. 3 and 4, which is discussed hereinafter,which would seem to be incompatible with the lack of such eccentricityin a conventional magnetic bearing. Thus, these eccentricitydifferences, wherein the natural rotational center of a rotor within amagnetic bearing would be different from its natural rotational centerwithin a foil bearing, would seem to rule out a hybrid use of both afoil bearing part and a magnetic bearing part.

If a hybrid journal bearing were to comprise magnetic control coils onopposite sides of the bearing, this would thus provide right and leftmagnetic planes. For a large load capacity, a foil journal bearing tendsto have a length to diameter ratio close to 1. This would make thehybrid journal bearing long in the axial direction with the result thatan undesirably large bending mode node could exist inside the hybridbearing. If all four bearing coils (quadrants 1 and 3, both right andleft magnetic planes, for vertical control; or quadrants 2 and 4, bothright and left magnetic planes, for horizontal control) for control inone direction (vertically or horizontally) were connected to be actuatedwith one bi-polar power amplifier to achieve a push-pull control action,sensor non-collocation may undesirably cause the magnetic bearingactuator to produce forces which are improperly phased with respect tothe shaft motion that is to be controlled.

The benefits of a hybrid foil-magnetic thrust bearing are considered tobe similar to those of a hybrid foil-magnetic journal bearing. Thus, thehybrid thrust bearing for high-speed and high-temperature applicationswould carry more load per pound of bearing weight than a conventionalthrust active magnetic bearing. The hybrid thrust bearing would havesuperior dynamic characteristics because the foil part, inherently ahigh speed bearing part, and the magnetic part, which with solid coresperforms well at low frequencies, would tend to complement each other.The foil part, since it would not take up any load at start-ups andshut-downs, would not need a coating or its coating, if provided, shouldlast a long time. Furthermore, at high speeds, the foil part would beable to take over and prevent bearing catastrophe in case of electric orcontrol failures.

At the operating speed, the load on a thrust bearing varies from aminimum or normal thrust load to a maximum due to, for example, acompressor surge. The hybrid thrust bearing should be able to take anyload amount within its capacity without “thinking” (performing off-linelogic calculations and making decisions in a supervising controller tore-set parameters) since there may not be enough time to do the“thinking”.

It is accordingly an object of the present invention to provide asuitably controllable hybrid foil-magnetic bearing.

It is another object of the present invention to provide control of ahybrid foil-magnetic journal bearing which is substantially free of theeffects of a bending mode node resulting from magnetic planes onopposite sides of the bearing and of sensor non-collocation.

In accordance with the present invention, there is provided a suitablycontrollable hybrid foil-magnetic journal bearing wherein the foil andmagnetic bearing parts share the load at a predetermined speed. Inaccordance with one embodiment thereof, foil and magnetic bearingstiffnesses are continuously re-calculated and new referenceeccentricities established therefrom and inputted to a controller tothereby provide control of steady state load sharing in the bearingwherein the control reference of the magnetic bearing part follows theeccentricity of the foil bearing part. In accordance with anotherembodiment thereof, an adaptive filter is provided to filter outvibration signals at the frequency coincident with the rotor speed fromcontrol current so that the control current need not be re-calculated atdifferent rotor spin speeds and, thus, so that load sharing may beachieved quickly without “thinking.” Alternatively, the integral controlcoefficient may be selected such that the magnetic part static stiffnessis reduced, preferably gradually, at increased rotor speed or speedsthereby allowing the rotor center to “seek” the foil bearing part centerat the respective rotor speed so that load-sharing may be achievedautomatically or without “thinking.”

In order to provide control of the journal bearing which issubstantially free of the effects of a bending mode node resulting frommagnetic planes on opposite sides of the bearing and of sensornon-collocation, in accordance with the present invention, for each axis(vertical and horizontal), a first signal is outputted, in response toinput of the shaft radial position from a first sensor on a first sideof the bearing housing, to a first control coil means for regulatingflux on the first side of the housing, and a second signal is outputted,independently of the outputting of the first signal, in response toinput of the shaft radial position from a second sensor on a second sideof the bearing housing, to a second control coil means for regulatingflux on the second side of the housing.

In order to provide a suitably controllable hybrid foil-magnetic thrustbearing, in accordance with the present invention, the foil bearing andmagnetic bearing parts are located on opposite sides of the thrustrunner for sharing the load. In accordance with one embodiment, controlcurrent is re-calculated as rotor speed changes. Alternatively, theintegral gain may be selected such that, regardless of rotor speed, thecontrol current will not need to be re-calculated at different rotorspeeds so that load-sharing may be achieved automatically and thusquickly without “thinking.”

The above and other objects, features, and advantages of the presentinvention will be apparent to one of ordinary skill in the art to whichthe present invention pertains in the following detailed description ofthe preferred embodiments of the present invention when read inconjunction with the accompanying drawings wherein the same referencecharacters denote the same or similar parts throughout the severalviews.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a hybrid foil-magnetic journalbearing which embodies the present invention.

FIG. 2 is a schematic view illustrating a control system for thebearing.

FIG. 3 is a graph illustrating a change in bearing eccentricity.

FIG. 4 is a graph of quadrant 2 of FIG. 3 illustrating change in bearingeccentricity as speed increases.

FIG. 5 is a schematic view, similar to that of FIG. 2, illustrating acontrol system for the bearing of FIG. 2 in accordance with analternative embodiment of the present invention.

FIG. 6 is a representative graph of magnetic bearing normalizedstiffness amplitude in relation to normalized frequency.

FIG. 7 is a representative graph of the corresponding phase angle forthe graph of FIG. 6.

FIG. 8 is a load sharing flow diagram for the bearing.

FIG. 9 is a representative graph of bearing stiffness coefficientsrelative to frequencies.

FIG. 10 is a representative graph of bearing damping coefficientscorresponding to the bearing stiffness coefficients of FIG. 9.

FIG. 11 is a representative graph illustrating the obtaining of thebearing natural frequencies.

FIG. 12 is a schematic diagram illustrating a sensorless control circuitfor either a journal or a thrust foil-magnetic hybrid bearing inaccordance with an alternative embodiment of the present invention.

FIG. 13 is a schematic sectional view, taken along lines 13—13 of FIG.14, of a hybrid foil-magnetic journal bearing in accordance with analternative embodiment of the present invention.

FIG. 14 is a schematic longitudinal sectional view of the bearing ofFIG. 13 and illustrating a control system therefor in accordance with analternative embodiment of the present invention.

FIG. 15 is a schematic half longitudinal sectional view of a hybridfoil-magnetic thrust bearing in accordance with an alternativeembodiment of the present invention.

FIG. 16 is a graph of rotor position and of magnetic bearing load duringrotor acceleration during a test of a hybrid bearing similar to thatshown in FIG. 1.

FIG. 17 is a graph showing rotor response during simulated failure ofthe magnetic bearing part of the hybrid bearing of FIG. 16 and recoveryof the magnetic bearing part for return to hybrid bearing operation.

FIG. 18 is a graph similar to that of FIG. 3 illustrating differences ineccentricities between a foil journal bearing and a hybrid journalbearing as illustrated in FIG. 5.

FIG. 19 is a graph for the purposes of explaining the differences ineccentricities illustrated in FIG. 18.

FIG. 20 is a schematic view similar to that of FIG. 2 of anotheralternative embodiment of the control system for a hybrid foil-magneticjournal bearing.

FIG. 21 is a schematic view similar to that of FIG. 15 of anotheralternative embodiment of the control system for a hybrid foil-magneticthrust bearing.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, there is illustrated generally at 20 ahybrid foil-magnetic journal or radial bearing for rotatably receiving arotor or shaft 22 (the shaft being shown with an exaggerated smalldiameter, it being understood that the shaft diameter approaches that ofthe bearing, i.e., the clearance typically being only on the order ofthousandths of an inch). The bearing 20 includes a generally cylindricalhousing 24 which receives the rotor 22. A foil bearing means or part 21includes a suitable thin flexible smooth sheet or foil 26 which isdisposed between the housing 24 and the rotor 22 and faces the rotor 22.One edge of the foil 26 is suitably anchored to the housing 24 as atsuitable anchoring means 28 and extends therefrom substantiallycircumferentially around the rotor 22 and terminates in a free ortrailing edge 30. The rotor 22 is illustrated in FIG. 1 to rotatecounterclockwise, as illustrated at 32. Disposed between the housing 24and the foil 26 are a series of, for example, three circumferentiallyspaced elements or sheets 34 each having a corrugated shape to define aplurality of circumferentially spaced ridges 36 to engage the foil 26and a corresponding plurality of circumferentially spaced troughs 38 toengage the housing 24 to thereby provide a compliant spring support forthe foil, i.e., resiliently bear or support the foil 26. The leadingedges of the corrugated supports 34 are anchored to the housing 24 as byanchoring means 40, and the supports 34 extend therefromcircumferentially to trailing free edges 42.

Elastic deflection of the foil 26 generates load-carrying hydrodynamicfilms. As speed increases, the foil 26 and corrugated support sheets 34are automatically forced radially outward, forming a converging wedge,which is an optimum shape for the desired hydrodynamic action and whichis achieved without the need for complex and expensive machining.Converging or lobing effects due to the bumps or ridges 36 become morepronounced as a function or speed and load thereby increasing bearingload capacity. The self-generated lobing effects also assist inenhancing rotor system stability through a reduction in cross-coupledstiffness and increased damping. The compliant surface readilyaccommodates shaft centrifugal growth with minimal loss of load capacityand without increased load complexity.

The corrugated supports may be constructed to provide the bearingstiffness needed to meet specific system dynamic requirements. Forexample, the corrugated supports 34 are illustrated in FIG. 1 tocomprise a nested or double bump configuration which includes an outercorrugated sheet 44 and an inner corrugated sheet 46, whichconfiguration is provided to achieve stiffness that varies with load.Such variable stiffness is provided to extend the operating range of afoil bearing. Low stiffness, provided by only the ridges 36 of the innercorrugated support 46 supporting the foil 26 when forces acting on itare low, aids in developing a converging wedge quickly causing filmpressures that will lift the rotor off the foil 26 quickly at lowspeeds. At high speeds, the higher generated gas film pressures mayrequire the stiffer supporting structure provided when the higher forcescause the ridges 36 of the inner corrugated sheet 46 to be engaged bythe ridges 36 of the outer corrugated sheet 44 to accommodate theoperating loads such as those due to rotor imbalance. Since, as will bediscussed hereinafter, the load will normally be borne by magnetic meansat start-up and low speeds, it may not be considered necessary to havethe low stiffness in which case a single high stiffness corrugatedsupport sheet may be provided. However, it still may be desirable tohave low stiffness as a back-up.

The foil bearing part 21, comprising the foil 26 and its supports 34,may be constructed in various other ways, taking into consideration theparticular requirements of the system being developed. For example, anindividual foil may be provided for each of the corrugated supports 34,or a single corrugated support may be provided for the foil 26. Foranother example, the foil support structure may comprise a plurality ofcircumferentially extending side-by-side rows each having a plurality ofcorrugated elements wherein ends of the elements in each row is offsetcircumferentially from ends of the elements in adjacent rowssymmetrically from the radial centerplane of the bearing assembly tothereby provide a herringbone pattern effect, as described in theaforesaid U.S. patent application Ser. No. 08/827,203, which isincorporated herein by reference, for enhancing the self-pumping actionof the bearing. Various other foil bearing constructions which may beincorporated, as suitable, in the bearing of the present invention aredisclosed in the patents and patent applications incorporated byreference relative to foil bearings.

The hybrid bearing 20 also includes a magnetic bearing means or part 51which comprises a stator 52 wound with coils 54 to create a magneticfield and ferromagnetic laminations, illustrated at 55, on the rotor 22to interact with the stator magnetic field to effect movement of therotor 22 radially in the direction in which the magnetic field appliesforce for movement of the rotor 22. Thus, a first pair of the coils 54 aand 54 c are diametrically opposite each other for effecting movement ofthe rotor 22 in a first direction, illustrated at 58, such asvertically, and a second pair of the coils 54 b and 54 d arediametrically opposite each other in a plane at right angles to theplane of the first pair for effecting movement of the rotor in a seconddirection, illustrated at 60, such as horizontally, which is normal tothe first direction 58. Bias currents, illustrated at 56, are applied tothe coils 54 to support static loads and set up an operating flux fieldfor linearized control. The bias currents 56 form a flux fieldequivalent to a negative spring with the result that the bearing 51 isinherently unstable and must be stabilized to function suitably.Stability is established by a negative feedback control of the rotordisplacements wherein control currents, illustrated at 62, are providedto modulate the bias currents 56 to create stiffness and damping and tocreate the desired stability.

The magnetic bearing part 51 may be constructed in various other ways,taking into consideration the particular requirements of the systembeing developed. For example, while the bearing part 51 is illustratedas being homopolar, it should be understood that it may have aheteropolar configuration. Various other constructions are disclosed inthe patents and patent application incorporated by reference relative tomagnetic bearings.

While the hybrid bearing 20 is illustrated in FIG. 2 to be of a nestedconfiguration wherein the foil part 21 is embedded within the magneticpart 51, it should be understood that the hybrid bearing 20 may beotherwise suitably configured such as, for example, wherein the foilpart 21 is placed outside/beside the magnetic part 51, or, for anotherexample, wherein the magnetic part 51 is placed on one or both sides ofthe foil part 21.

FIG. 2 illustrates a control system 64 for the pair 54 a and 54 c ofcoils for vertical control, it being understood that a similar controlsystem is provided for the horizontal control coils 54 b and 54 d. Aposition sensor 66 is suitably disposed to provide rotor position in therespective magnetic plane, i.e., vertically for coils 54 a and 54 c, itbeing understood that a horizontal position sensor is provided for coils54 b and 54 d. It should however be understood that other forms of rotorposition sensing (directly or indirectly) may be provided such as, forexample, flux feedback within the gap between the rotor and the magneticbearing poles. Sensor 66 provides a signal of measured rotordisplacement in the respective direction (vertically for coils 54 a and54 c) via line 68 to summer 70 where this signal is combined with areference, i.e., the desired rotor position at that point in time, whichis inputted thereto from a supervising controller 65 via line 63. Theresulting summed signal is then transmitted via line 72 to a sensorconditioner 74 of a feedback control loop 76. The purpose of the sensorconditioner 74 is to establish the dynamic motion or displacement of therotor and provide a suitable signal for a PID (proportional, integral,and derivative control) controller 78 to which the conditioned signal isthen sent. An input to P (proportional), illustrated at 80 in FIG. 14(the controllers in FIGS. 2 and 14 being identical), of controller 78for dynamic stiffness control is the time varying position signal,illustrated at X in FIG. 14. Control currents based solely on rotorposition, while providing dynamic stiffness control, are considered tobe inadequate alone to control the rotor at resonances or criticalspeeds. For reliable rotor control, it is considered that both rotorposition and its rate of change need to be corrected. Thus, damping orvelocity control, which is achieved by adding rotor velocity feedback tothe current control, is also considered to be needed. In addition todynamic stiffness and damping, basic rotor position error feedback isalso considered to be required to statically center the rotor. Thecontroller 78 is thus provided to sum in summing circuit 86 theproportional signal 80 for dynamic stiffness control, the integral, I,of the position signal error, illustrated at 82, for static stiffnesscontrol, and the derivative, D, of the time-varying position signal,illustrated at 84, for damping, in accordance with principles commonlyknown to those of ordinary skill in the art to which this inventionpertains, and to output the signal of the summed information via lines92 a and 92 c to power amplifiers 88 a and 88 c for coils 54 a and 54 crespectively after being suitably gain and phase compensated, asillustrated at 90. The signal i is added to bias current I for deliveryto one of the coils 54 a and subtracted from the bias current I fordelivery to the other coil 54 c for increasing the flux from one coiland decreasing the flux from the other coil for effecting a movement inthe desired direction radially vertically of the rotor 22. Therespective signals are amplified in amplifiers 88 a and 88 c beforepassing to the respective coils. The feedback control circuitry as sofar described with reference to FIG. 2 is conventionally known in theart and is described and illustrated in the aforesaid U.S. Pat. Nos.5,202,824 and 5,084,643 which, as previously discussed, are incorporatedherein by reference. Since a PID controller as well as its use isconventionally known in the art, it will not be described in furtherdetail herein. It should however be understood that other suitablecontrol means may be employed such as, for example, state space,non-linear, adaptive, and/or fuzzy logic control methods.

For operation of the hybrid bearing 20, it is preferably controlled sothat the magnetic bearing part 51 initially centers the rotor 22 toeliminate rubbing between the foil 26 and rotor 22 during low speeds andstart-up, then release the load gradually to the foil bearing part 21,and provide synchronous balancing forces and additional damping forenhanced high speed stability while a share (or all) of the load istaken by the foil bearing part 21.

FIG. 3 illustrates the 4 quadrants 1, 2, 3, and 4 within the magneticbearing clearance circle 91 with the center illustrated at 92, the foilbearing clearance circle being shown at 93, i.e., being smaller than themagnetic bearing clearance to prevent contact between the rotor 22 andthe stator laminations 55, and FIG. 4 is an enlarged view of quadrant 2.

High static stiffness provided by the magnetic bearing part 51 keeps therotor 22 centered within the bearing 20, i.e., with its center at center92. The dynamic stiffness of a magnetic bearing can be represented bythe following equation in a normalized form for the PID control system78:

K/K _(m)=(G _(p) G _(a) K _(i) /K _(m))[C _(p) +C _(i)/(τ_(i) S+1)+C_(d) S/(τ_(d) S+1)]−1

where

K=magnetic bearing stiffness, lb/in

K_(m)=magnetic stiffness, lb/in, function of bias current

K_(i)=current stiffness, lb/Amp, function of control current

G_(p)=displacement sensitivity, V/in

G_(a)=power amplifier sensitivity, Amp/V

C_(p), C_(i), C_(d)=P, I, and D gains respectively

τ_(i), τ_(d)=time constant of integrator and differentiator,respectively, sec.

The above equation indicates that the magnetic bearing stiffness anddamping can be varied by three gains and two time constants, i.e.,proportional, integral, and derivative gains and the integrator anddifferentiator time constants. Suitable lead-lag or notch filtercircuits, illustrated at 90, are, in accordance with the presentinvention, added in series with the PID controller 78 (to the outputthereof) to provide gain and phase compensation at resonant frequenciesnot covered by the PID circuit 78. FIG. 6 is a graph of magnetic bearingstiffness amplitude at different normalized frequencies. In order toachieve the desired frequency response, the roll-off frequencies of theintegrator and differentiator are set, for example, respectively at 0.1and 10, and the roll-off frequency of the power amplifier 88, notincluded in the above equation, is set, for example, at 20. The graphshows one D.C. stiffness at 94 for an integrator gain of 4 and anotherat 96 for zero integrator gain. Thus, by varying the magnetic bearingD.C. stiffness, more or less load can be moved onto or off of the foilbearing 21, because varying the DC stiffness will change the runningposition of the rotor. While reducing stiffness so that the load canpartially (or completely) be carried by the foil bearing part, it isimportant that the bearing system stability not be adversely impacted.One measure of bearing system stability is illustrated in FIG. 7, whichshows the corresponding phase angle of the dynamic stiffness for thechanges in integrator gain. A positive phase angle (damping) isindicative of stability at a given frequency. Since the phase angle isshown to be positive at the rotor critical frequencies, includingrigid-body and bending modes, in the range between about 1 and 12(normalized), this is thus indicative of rotor-bearing system stability.Magnetic bearing D.C. stiffness reduction may also be accomplished byreducing proportional gain.

It is the normal function of a purely magnetic bearing to keep the rotorcentered, i.e., with its center at the bearing center, illustrated at92. However, as seen in FIG. 4, wherein the line 98 generally connectsthe loci, illustrated at 100, of the rotor center at various loads for a35 mm shaft in counterclockwise rotation and with 1.5 mil clearance,which is typical of foil bearings, the rotor center is generally atlocus under static load and moves generally downwardly at an angle tothe vertical of between about 20 and 30 degrees as load increases, asillustrated by arrow 104. The eccentricity locus in FIG. 4 indicatesthat a foil bearing may be loaded to an eccentricity ratio of 2 to 3.For a radial clearance of 2 mils, this translates into an operatingeccentricity of 4 to 6 mils from the hybrid bearing center, assumingthat the foil and magnetic elements are concentric. A magnetic bearingtypically has a radial air gap of 20 to 30 mils. Therefore, the changein foil bearing operating eccentricity represents a large portion of thegap, and non-linearity of the magnetic bearing is expected. In order tohandle such non-linearity and thus perform suitably for smooth loadsharing and changes in load sharing as speed is increased and decreased,in accordance with the present invention, the steady state load sharingratios are established throughout the speed range, i.e., all of the loadtaken by the magnetic bearing at lift-off, gradually changing the loadsharing ratio as speed increases. Establishing the load sharing ratiosis important because the dynamic properties of both the foil andmagnetic bearings depend on their operating steady state load. However,first, the steady state load magnitude and direction as a function ofspeed should be determined.

Referring to the flow chart of FIG. 8, which may be considered to be acomposite of design procedure and active control, there is showngenerally at 65 in FIGS. 2 and 8 a supervising control circuit fordetermining the steady state load magnitude and direction as a functionof rotor speed, wherein the foil and magnetic bearing part stiffnessesare continually re-calculated based on rotor speed, as illustrated at 67and new reference eccentricities are continually re-calculated therefromfor input to summer 70, as illustrated at 69. The process is morespecifically illustrated in FIG. 8. Thus, the foil and magnetic bearingdata are stored in order to conduct the hereinafter describedcalculations, as illustrated at 110 and 112 respectively in FIG. 8. Inaccordance with the present invention, the reference point is put at themagnetic bearing center 92, as illustrated at 114, and, at each of thepredetermined speeds at which load sharing is to be performed, determinethe total static load and direction with the magnetic bearing taking allof the steady state load, as illustrated at 116. Using the stored data110, determine the eccentricity and attitude angle of the foil bearingfor a predetermined share of the load, as illustrated at 118. Then, movethe magnetic bearing reference (which is the same as rotor center) tothe calculated eccentric location, illustrated at 120 in FIG. 3,predicted by the foil bearing, and, once the rotor center has been sore-located, measure the currents in the magnetic bearing coils and therotor true location, and calculate the steady state load actuallysupported by the magnetic bearing, as illustrated at 122. If necessary,the integral gains should be adjusted to make the steady state loadsharing correct, as illustrated at 124. Then, using the stored magneticbearing data 112, recalculate the current stiffness, magnetic stiffness(because both coil steady state currents and air gaps will have changed)and the dynamic stiffness as a function of excitation frequencyaccording to the existing PID gains, as illustrated at 126. Next, usingstored foil bearing data 110, calculate the stiffness and damping of thefoil bearing for the steady state load, and add these dynamiccoefficients to those of the magnetic bearing, as illustrated at 128.Then, check the adequacy of the total dynamic properties in terms ofrotor stability, and, if these are not considered suitable, adjust thePID controller gains to improve them, as illustrated at 130. Thestiffnesses of the foil and magnetic bearing parts are thus selected, asillustrated at 128 and 126 respectively, to allow the load sharing tosuccessfully take place.

The following example of the above load sharing process is forillustrative purposes only and not for purposes of limitation. Inaccordance with this example, a hybrid foil-magnetic bearing has thefollowing specifications:

Foil part Magnetic part Diameter (inches) 4.0 4.0 Length (inches) 3.01.0 Radial clearance (in.) 0.002 0.0225 Share of static load (lb.) 21.014.0

The share of static load is desirably selected to be such as to minimizetotal bearing volume and weight as well as increase the load capacity ofthe bearing with a minimum of additional weight. The load sharing isperformed at a rotor speed of, for example, about 10,000 rpm, which isselected because it is representative of expected operating conditionsof, for example, a gas turbine engine rotor speed, and is in a speedregion (low enough speed) where the foil bearing actively supports loadand is thus useful. At this speed, the foil bearing non-dimensional data110 are as follows:

ε φ W/P_(a)R² K_(xx)/P_(a)R² K_(xy)P_(a)R² K_(yx)/P_(a)R² K_(yy)/P_(a)R².200 60.0 .0975 .389 .437 .105 .309 .400 46.0 .2403 .590 −.658 .187 .620.600 32.0 .4774 .880 −.819 −.049 1.285 .700 26.0 .6466 1.041 −.861 −.1721.699 .800 20.0 .8532 1.151 −.842 −.270 2.121 .900 16.0 1.0951 1.295−.800 −.342 2.498 .950 15.0 1.2276 1.356 −.765 −.359 2.657

For the load of 21 lb., the foil bearing eccentricity, attitude angle,and stiffnesses are calculated by interpolation from the above table ofnon-dimensional data 110 to be as follows:

ε φ e e_(x) e_(y) .4925 39.5 .9851E − 03 .6269E − 03 −.7599E − 03 K_(xx)K_(xy) K_(yx) K_(yy) .2173E + 05 −.2197E + 05 2334. .2783E + 05

Initially, the magnetic bearing is treated as supporting all of the 35lb. (21.0 plus 14.0) static load, and its reference is set at X=Y=0, inaccordance with step 114. The following magnetic bearing parameters arenow obtained: $\begin{matrix}\quad & \quad & {Top} & {Bottom} & {Right} & {Left} \\{{Initial}\quad {bias}\quad {currents}\text{:}} & {I_{10},I_{30},I_{20},{I_{40} =}} & {{4.200\quad A},} & {{3.000\quad A},} & {{3.000\quad A},} & {3.000\quad A}\end{matrix}$

Now the rotor center as well as the magnetic bearing reference centerare shifted to eccentric point 120 (FIG. 3), in accordance with step122. With less load (i.e., 14 lbs.) to support by the magnetic bearing(the remainder of the 35 lb. load being supported by the foil bearing),the new magnetic bearing bias. currents, air gaps, and magnetic quadrantforces are calculated, in accordance with step 126, to be as follows:$\begin{matrix}{I_{4} = {3.084\quad A}} & \begin{matrix}{I_{1} = {3.963\quad A}} \\{I_{3} = {3.237\quad A}}\end{matrix} & {I_{2} = {2.916\quad A}}\end{matrix}$ $\begin{matrix}{g_{1},g_{2},g_{3},{g_{4} =}} & {.0233} & {.0219} & {.0217} & {.0231} & ({in}) \\{F_{1},F_{2},F_{3},{F_{4} =}} & 59.22 & 36.27 & 45.22 & 36.27 & ({lb})\end{matrix}$

Without changing the PID gains, the magnetic bearing stiffness anddamping coefficients are calculated over a frequency range of 0 to 200Hz. It should be noted that these coefficients are frequency dependentwhile those of the foil bearing are not. Adding the coefficients for thefoil and magnetic bearings together, the stiffness and dampingcoefficients for the hybrid bearing are shown at 140 and 142respectively in FIGS. 9 and 10 respectively. In accordance with step130, in order to determine the bearing natural frequencies, Kxx, Kyy,and Mw2 are graphed, as illustrated at 144, 146, and 148 respectively inFIG. 11. As seen in FIG. 11, the intersection between Kxx and Mw2 is at112 Hz. The rotor speed of 10,000 rpm or 167 Hz. is far above thisnatural frequency, and, therefore, the bearing stiffness and dampingcoefficients are considered to be acceptable. A more precise calculationof damped frequencies yields the following eigenvalues: −124±j629 and−241±j645 rad/sec. The corresponding modal frequencies are 100 and 103Hz., not too far away from those derived from FIG. 11. It should benoted that the foil bearing damping has not been included in the abovecalculations since these two modes are reasonably damped, and moredamping thereof can be achieved by adjusting the PID gains.

The present invention is not limited to the above load-sharingprocedure. In an effort to provide a more simple means of controllingthe bearing 20 so as to minimize the “thinking” necessary to controlbearing performance, alternative means of controlling the hybrid bearing20 may, for example, include predetermined speed dependent controlschedules, prescribed rotor displacement limits, and an adaptive controlmethod. Alternatively, the control of the hybrid bearing 20 may beaccomplished through means that controls the magnetic bearing currentaccording to a speed dependent gain-phase schedule such that themagnetic bearing takes less rotor load as a function of speed. Thisschedule would be implemented to match the increasing load capacity ofthe foil with increasing speed. Another embodiment may be that thecontrol process be such that the rotor orbit and center position bemaintained within prescribed limits and that the controller allow thefoil bearing to carry as much load as possible while still operatingwithin those prescribed limits. For example, the controller may restrictrotor position not to exceed 60% of the foil bearing operating clearancecircle and that the rotor dynamic motions or orbit not exceed 30 to 40%of the foil bearing clearance circle. An adaptive control method mayalternatively be employed, as described hereinafter with reference toFIG. 5. Thus, while operating at a particular speed the controller maybe caused to reduce the magnetic bearing stiffness by reducing theelectrical current so that the foil bearing carries a majority of theload and the shaft center finds its natural eccentricity position forthe operating speed. From this eccentric position, the magnetic bearingstiffness (i.e., current) and load share are increased until the desiredconditions are achieved. Combinations of the above control methods maybe employed. For example, the control system may incorporate both theprescribed limits and the adaptive control method. In this system, theadaptive method may be used to determine the optimum operatingconditions within the prescribed limits. These optimum conditions may beminimum control current, minimum dynamic orbit, or minimum vibrations atparticular system natural frequencies. Load-sharing based on integralgain modification is described more specifically hereinafter withreference to FIGS. 20 and 21. Such alternative control means orprocedures are meant to come within the scope of the present invention.

Referring to FIGS. 13 and 14, there is shown generally at 200 a hybridfoil-magnetic bearing having a foil bearing part 201 including a housing202, a smooth thin flexible foil, illustrated at 204, for facing therotor 22, and a corrugated sheet (not shown but similarly as illustratedat 34 in FIG. 1) for supporting the foil 204, the foil bearing part 201being similar to the foil bearing part 21.

The foil bearing part 201 is nested within a homopolar active magneticbearing part 211 comprising ferromagnetic material 212 about which iswound a bias coil 214 to form an electromagnet for interaction withferromagnetic laminations 216 on the rotor 22, similarly as discussedfor bearing part 51. This homopolar bearing part 211 has right and leftside homopolar planes, illustrated at 218 and 220 respectively, andcontrol coils 222 are provided on each plane 218 and 220.

In accordance with the present invention, the independently poweredsingle bias coil 214 is provided for both bearing halves 217 and 219(corresponding to bearing planes 218 and 220 respectively) located onopposite sides axially of the foil bearing part 201 to provide the basicmagnetic flux required by both magnetic bearing halves 217 and 219 tothereby achieve reduced cost and complexity.

For a large load capacity, a foil bearing tends to have a length todiameter ratio close to one, which makes the hybrid bearing long in theaxial direction and which therefore may result in a bending mode nodeinside the bearing. When a single rotor position sensor is used for eachaxis (vertical and horizontal) and the 4 coils (for example, those inquadrants numbered 1 and 3 in FIG. 13, both right and left for thevertical axis) are connected for push-pull control action, improperlyphased forces with respect to rotor motion (sensor non-collocation) maybe produced by the magnetic bearing actuator due to the sensor not beinglocated directly adjacent to or in-line with the bearing axial center.In order to prevent or reduce the effects of sensor non-collocationwhile also providing improved reliability and control flexibility andthe ability to control rotor system vibrations even when a rotor naturalfrequency node (location of very small amplitude relative to the bearingcenterline) occurs within the axial length of the bearing, in accordancewith the present invention, a pair of rotor position sensors 224 and 226are provided for each of the left and right sides 220 and 218respectively for the vertical axis (quadrants 1 and 3) and another pairof rotary position sensors (not shown) are provided for each of the leftand right sides 220 and 218 respectively for the horizontal axis(quadrants 2 and 4), and the left and right bearing sides 220 and 218for each of the vertical and horizontal axes are controlledindependently by means of the respective sensors. The control circuitryfor the horizontal axis is similar to that shown in FIG. 14 for thevertical axis, as described hereinafter. Thus, referring to FIG. 14,actual data received from sensor 224 for the vertical axis on the leftside 220 of the bearing is sent via line 132 to a comparator 156 then(assuming for the moment no other input thereto) to the left sidevertical PID controller 78 a, where a suitable signal is generated, inaccordance with the principles previously discussed, amplified by poweramplifier 136, and sent to the pair of control coils 222 for quadrants 1and 3 of the left bearing side 220 for exerting force along the leftbearing side 220 for effecting movement of the rotor vertically.Similarly, actual data received from sensor 226 for the vertical axis onthe right side 218 of the bearing is sent via line 142 to anothercomparator 156 then (assuming for the moment no other input thereto) tothe right side vertical PID controller 78 b, where a suitable signal isgenerated independently of the signal generated by controller 78 a, inaccordance with the principles previously discussed, amplified by poweramplifier 146, and sent to the pair of control coils 222 for quadrants 1and 3 of the right bearing side 218 for exerting force along the rightbearing side 218 for effecting movement of the rotor vertically.Although an increased number of power amplifiers, sensors, andcontrollers is required, each independent bearing side control axisrequires less power, and use of multiple power supplies/amplifiersadvantageously provides redundancy in the system. Thus, even if oneamplifier fails, the other bearing side for the same axis should remainactive and energized. Total cost of the control system should besubstantially less than double that of a system having a single sensorfor an axis since smaller individual components should be lessexpensive.

A supervising or hierarchal controller 65 is provided for adjusting theload-sharing similarly as discussed with respect to FIG. 2.

In order to use both left and right bearing sensors for each axis in thecontrol of both left and right magnetic bearing sides 220 and 218respectively for enhanced system reliability, in accordance with apreferred embodiment of the present invention, the left side comparator156 for each axis (vertical and horizontal) also receives an input vialine 150 from observer 151, and, likewise, the right side comparator 156for each axis also receives an input via line 170 from observer 171. Theleft side observer 151 receives an input of actual data from right sidesensor 226 via line 153, and, likewise, the right side observer 171receives an input of actual data from left side sensor 224 via line 173.Observer 151 is a suitable neural net system, commonly known to those ofordinary skill in the art to which this invention pertains, whosefunction is to predict and output a signal to respective comparator 156via line 150 corresponding to predicted data of the left sensor 224based on input via line 153 of actual data from the other or rightsensor 226 and stored data of an analytical model of the system and ofthe feedback along respective line 155 of an accumulation of priorvalues of the respective comparator outputs along respective line 157(representative of actual data from sensor 224). For example, theobserver 151 may store in memory an accumulation of comparisons ofsignals along line 155 (representative of actual data from left sensor224) and actual data from the other or right sensor 226 and predict,based thereon, the actual data from the left sensor 224 when given vialine 153 the actual data from the other or right sensor 226. Duringnormal operation as this comparison information is accumulated, thepredictions by observer 151 of data from sensor 224 should approach theactual data from sensor 224 with the result that the feedback signalsvia line 155 should indicate only small differences between thepredicted value (along line 150) and the outputted value (along line157) of the data from sensor 224. As long as these differences are small(within a certain predetermined range), it can be assumed that thesensor 224 is given reliable data. Thus, if the difference is within therange, the respective comparator 156 is suitably programmed to output asignal along line 157 which is equal to the actual data from sensor 224,and the feedback thereof along line 155 will be added to the accumulateddata from which the observer 151 in the future predicts the data fromsensor 224. However, if the difference is outside the predeterminedrange (for example, the predicted value of data from sensor 224 via line150 indicates a high rotor motion or displacement based on the actualdata from the other sensor 226 yet the actual data from sensor 224 vialine 132 indicates that the rotor is not vibrating), then it may beassumed (based on experience that when a sensor becomes inoperative, itshows no rotor motion) that the sensor showing no rotor motion hasbecome inoperative in which case the comparator is programmed to acceptand output along line 157 the predicted data from sensor 224 as providedby observer along line 150. The observer 171 and its comparator 156 aresimilar in operation to that described for observer 151 and itscomparator 156. Thus, if the left sensor 224 for the left bearing sidefails, a signal is generated from the data from the right bearing sidesensor 226 that simulates the left bearing motion (as well as phaseinformation) for delivery to the power amplifier 136 for control of theleft bearing side 220. Generation of this observed and simulated timeseries data requires a priori knowledge of the rotor-bearing systemdynamics. Thus, analytical predictions of the rotor bearing systemperformance are established during development of the system and storedin memory for use with the accumulated feedbacks through lines 155 formaking the predictions.

It should be understood that, while the control system of FIG. 14 hasbeen shown in connection with a nested configuration wherein the foilbearing part is nested within the magnetic bearing part, it may also bemodified for use with a side-by-side configuration wherein separatemagnetic bearing parts, whether homopolar or heteropolar, are located onopposite sides of the foil bearing part.

A control procedure may alternatively be provided which does not makecontrol of the left and right magnetic bearing sides 220 and 218respectively independent, wherein alternative methods for redundancyand/or improved rotor system performance may be influenced by eachother. For example, during a change in unbalance due to rotor damage, itmay be advantageous to allow the left side 220 of the bearing to beinfluenced by the right side 218 of the bearing to minimize totalbearing loads, or, if the rotor is moving inside the bearing with bothends having the same phase relationship, only one sensor may be neededto control the rotor orbit.

FIG. 16 illustrates the results of a test to confirm that both the foiland magnetic parts of a hybrid bearing in accordance with the presentinvention support rotor loads. FIG. 16 shows both rotor verticalposition at the magnetic sleeve, at 402, and magnetic bearing controlforce (which equates to load), at 404, while the rotor speed wasincreased from 0 to 30,000 rpm under a load of 80 pounds. At the startof the rotor acceleration, the magnetic bearing part is shown, asillustrated at 406 by the magnetic control force, to support almost theentire 80-pound load. As speed is increased and hydrodynamic pressuresare generated in the foil bearing part, the rotor “lifts off” the topfoil of the foil bearing part, as illustrated by the sudden change inrotor position and magnetic bearing force, as illustrated at 408 and 410respectively. The almost constant magnetic force and rotor position forthe next five data points, while not following the overall trend ofdecreasing magnetic force with speed may be due to sensor error or tothe hydrodynamic pressures at rotor lift-off speed being small comparedto the higher speeds, and the rotor may have only been supportedintermittently by the hydrodynamic gas film pressures. After theseintermediate 5 data points, the magnetic force is shown to continue todecrease to the point, illustrated at 412, that the magnetic bearingpart is only supporting approximately 15 pounds of the total static anddynamic shaft load at 30,000 rpm. Similarly, the rotor vertical positionis shown to follow a consistently increasing rise with speed. Thus, FIG.16 is consistent with load-sharing by the foil and magnetic bearingparts of the hybrid bearing.

In the event that some component of the magnetic bearing part fails,electrical power is lost, or if transient loads exceed the load carryingcapability of the magnetic beating part, the hybrid bearing must be ableto continue to support the load. FIG. 17 shows the results of tests at15,000 and 20,000 rpm, illustrated at 420 and 422 respectively, toconfirm the ability of the foil bearing part to support the rotor duringsuch transient events, wherein electromagnetic failures and recoverieswere simulated. These lower speeds were used for the tests because theyrepresent the more severe condition for the foil bearing part, i.e., thebearing load to be assumed by the foil bearing part is lower at loweroperating speeds. Hybrid bearing operation wherein load is shared isillustrated at 424. At 426 is illustrated deactivation of the magneticbearing part. At 428 is illustrated operation on the foil bearing part.At 430 is illustrated reactivation of the magnetic bearing part. Itshould be noted that the rotor excursions occurring during thedeactivation and reactivation of the magnetic bearing part are less at20,000 rpm than at 15,000 rpm. This further indicates confirmation thatthe foil bearing part supports greater loads at higher speeds. Thus,this test indicates confirmation of the capability of the foil bearingpart to continue to support the rotor during transient events.

Referring to FIG. 15, there is shown generally at 300 a hybridfoil-magnetic thrust bearing comprising a foil bearing part 302 and amagnetic bearing part 304. Such a bearing for high-speed andhigh-temperature applications advantageously carries more load per poundof bearing weight than a conventional active magnetic thrust bearing.Such a bearing also has superior dynamic characteristics, because thefoil bearing part is inherently a high speed bearing and the magneticbearing part with solid cores performs well at low frequencies, and thefoil and magnetic bearing parts therefore complement each other.Advantageously, the foil coating is long-lasting since the foil bearingpart does not take any load at start-ups. At high speeds, the foilbearing part can advantageously take over and prevent catastrophe incase of electric or control failures.

The foil bearing part 302 includes a generally cylindrical housing 306having a central opening, illustrated at 308, in which the rotor 310 isreceived. A thin flexible smooth foil 312 is provided for facing therotor runner 314 for bearing the thrust thereof. A corrugated sheetmeans 316 having alternating ridges 318 and valleys 320 is interposedbetween the foil 312 and housing 306 to bearingly support the foil 312in its facing the runner 314 for receiving thrust load, as indicated byforce Ff, illustrated at 315. The foil bearing part 302 may beconstructed to various shapes and specifications depending on particularapplication for which it is to be used. For example, the corrugatedsheet means 316 may comprise two or more sheets providing variablestiffness, similarly as the corrugated sheet means 34 of the journalbearing 20 of FIG. 1. Various alternative embodiments as well as a moredetailed description of a suitable foil bearing part are disclosed inthe aforesaid patents and applications incorporated by reference withrespect to foil bearings.

The magnetic bearing part 304 includes a generally cylindrical core 322,composed of silicon steel or other suitable material and having acentral opening for receiving the rotor 310. A coil means 324 is held inplace in a notch 330 in the core 322 by a support structure 326 composedof a suitable non-magnetic material. A powder lubricated rub ring 328for facing and taking physical contact with the rotor runner 314 toprevent direct contact or rubbing of the magnetic cores in case ofover-load conditions is received in the notch 330 and protrudes slightlytherefrom. Interposed between the rub ring 328 and the support structure326 is a suitably stiff but damped material 332 for supporting the rubring. The coil means 324 is supplied with bias and control currents,similarly as described for bearing 20, to effect magnetic interactionwith the solid core ferromagnetic runner 314 to exert a flux field,illustrated at 334, which is variable by varying the control current,for bearing the runner 314, and receives input from a position sensor336. A more detailed description of such a magnetic bearing is containedin the aforesaid patents and application referring to magnetic bearings.

The hybrid thrust bearing 300 utilizes a PID controller 338 along with asensor conditioner 340, gain and phase compensator 342, and poweramplifier 344 all similar to the PID controller 78, sensor conditioner74, gain and phase compensator 90, and power amplifier 88 respectivelyof the hybrid journal bearing 20 for receiving thrust runner positioninput from the position sensor 336 and regulating control current to themagnetic bearing coil 322. At an operating speed, the foil bearing part302 on one side of the thrust runner 314 takes part Ff of the total loadFt, and the magnetic bearing part 304 on the other side of the thrustrunner 314 takes the remainder Fm of the total load Ft.

A supervising controller 346 is provided to effect the sharing of thethrust load to the maximum capacity of each of the foil and magneticbearing parts and to maintain suitable dynamic properties (stiffness anddamping) in accordance with the following discussion.

Assuming that the thrust load is smaller than Fm at start-up (whichshould normally be the case), the magnetic bearing part 304 takes all ofthe load and uses a high integral gain control to unload the foilbearing part at low speed, if necessary. At the operating speed, theload varies from a minimum or normal thrust load to a maximum due to,for example, a compressor surge. The load which can be assumed by thefoil bearing part 302 is rotor speed dependent, that is, as the rotorspeed increases, the foil bearing part 302 can assume greater thrustloads since thicker air films are generated by higher speeds, and it isthus desired that its share of the thrust load be increased withincreased rotor speed. The supervising controller 346 is provided tocontinuously calculate, based on inputs of rotor speed as inputtedthereto by sensor 331 or other suitable means, the runner position asinputted thereto by position sensor 336, magnetic force as inputtedthereto from amplifier 344, and stored information as to desired amountsof load-sharing at various rotor speeds, a runner position relative tothe foil bearing part for the respectively rotor speed, as illustratedat 333, i.e., the runner position at which it is considered that thebearing should operate optimally at the respective rotor speed. Forcecontrol (not to exceed the force capacity of the foil bearing part) mayalternatively be provided. After comparing the new runner position tothe foil thrust load limit and accordingly changing the runner positionif necessary so that the foil bearing part is not overloaded, asillustrated at 335, the runner position is then set, as illustrated at337, for delivery via line 339 to comparator 341 for comparing with theactual runner position signal obtained from position sensor 336 via line343 to output a signal for delivery to the PID controller for suitablymodifying the control current to the magnetic bearing part for movementof the thrust runner to the desired position for the particular rotorspeed.

The function of the supervising controller 346 may include intelligenton-line decisions on gain adjustment according to the displacement,temperature, speed, and other measurements and may also include aninitiation of a band-pass filter to eliminate synchronous axialvibration or a structure resonance excited by a blade-pass frequencysource.

It may however be desirable that the hybrid thrust bearing 300 be ableto take any load amount within its capacity and suitably share itwithout thinking (i.e., without the time consuming performance ofoff-line logic calculations and making decisions therefrom in thesupervising controller 346 or otherwise) since there may not be enoughtime to do the “thinking.” In other words, the load sharing shouldresult from an in-line signal variation on the fly so the needed signalchange is not slowed down by off-line calculations, somewhat akin to anatural reflex. In order to accomplish this, the following twoconditions should be satisfied. Firstly, at the operating speed andunder the foil bearing part capacity load Ff, the thrust runner shouldmove to close the gap between itself and the foil bearing part by anamount no greater than emax. This defines the maximum axial deflectionthat the thrust runner may traverse at the hybrid bearing capacity load.Secondly, at the same time (i.e., without the necessity to “think”), thegap between the runner 314 and the magnetic bearing 304 is increasedfrom a nominal value g0 to the maximum value g0+emax but should take itscapacity load Fm. To satisfy the latter condition, the static deflectionin the magnetic bearing part 304 should comply with the followingrelationship:

e _(max) ≈F _(m) /K _(dc)

and

K _(dc) =K _(i) G _(p) G _(a)(C _(p) +C _(i))−K _(m)

where

K_(i)=current stiffness

K_(m)=position stiffness

C_(p)=proportional gain

C_(i)=integral gain

G_(p)=displacement sensor sensitivity

G_(a)=power amplifier sensitivity.

The above equations indicate that if the static magnetic bearingsstiffness is set too high in the hybrid bearing 300, the magneticbearing part 304 may be flux-saturated before the foil bearing part 302can share the load. Thus, it may be necessary to reduce the integralgain to a lower value after a start-up or foil lift-off.

Another magnetic bearing relation which should be satisfied is asfollows:

F _(m) =f{(I _(b) +ΔI)/(g ₀ +e _(max))}²

where

f=magnetic force constant

I_(b)=bias current

ΔI=additional steady state or DC current

g₀=nominal magnetic air gap.

The amount of current Ib+delta I may be the design limit of the poweramplifiers or the flux saturation current of the magnetic core materialwith the air gap g0+emax.

Referring to FIG. 12, in order to eliminate the supervisory controller346 (as well as the PID controller) from the hybrid thrust bearingcircuitry in order that load-sharing may be achieved without “thinking,”as well as to provide a feedback control system for the hybrid thrustbearing which is inexpensive, robust, and reliable, the current to themagnetizing coils is controlled by velocity feedback control circuitry,illustrated at 710. Such a velocity feedback control system may also beprovided for a hybrid foil-magnetic journal bearing. This circuitry,which may be called “sensorless” control since it does not utilizeseparate sensors such as a rotor position or a rotor speed sensor,involves three separable functions, namely, velocity estimation,self-start, and velocity feedback control. The sensorless control has tobe initiated by a rotor motion. The self-start function is to providethis initial motion by injecting a current pulse into the coil 322. Thecurrent pulse in turn creates a force impulse to the rotor to kick itaway from either the foil or the magnetic bearing part, whichever it is“leaning” against. The velocity controller will then grab and keep therotor in the air gap. A distinctive feature of such a sensorless controlis that the rotor settles in the magnetic air gap in a directionopposite to the direction of thrust load increase. The aforesaid U.S.patent application Ser. No. 09/046,334 discloses such a control systemfor a hybrid thrust bearing with reference to FIGS. 4 and 5 thereof.

Velocity feedback control is based on the existence of a static forceequilibrium or balance position in the bearing clearance. This positionserves as an axial displacement control reference. The velocity feedbackcircuitry 710 is provided to regulate the supply of current fromamplifier 704 in order to create modulating magnetic forces to keep therotor 310 at this position. The balance or equilibrium position may, forexample, be slightly off to one side due to a static load applied on therotor 310, assuming that the electromagnets are equally strong. When thestatic load changes, the rotor 310 will automatically (without“thinking”) settle at a new balance position. The new balance positionwill, however, not be a stable one without the velocity feedback controlprovided by circuitry 710. This has been classically termed “unstableequilibrium.” Its like balancing a vertical stick from one's hand; thebottom of the stick must be moved around to keep the stick standing up.

It should be emphasized that the active thrust bearing static stiffnessis very different from, usually much higher than, its dynamic stiffness.In general, an active magnetic bearing stiffness is a function ofexcitation frequency.

The feedback control circuitry 710 comprises a velocity feedbackcontroller 716 and a velocity estimator 718 in addition to theself-starter 720. See H. M. Chen (an inventor of the present invention),“Design and Analysis of a Sensorless Magnetic Damper,” presented at ASMETurbo Expo, Jun. 5-8, 1995, Houston, Tex., 95GT180, as well as theaforesaid Chen patent. The velocity feedback controller contains apositive feedback loop 722 which may be called a zero force seekingloop. When the average rotor position is not at a static balance point,the rotor 310 will be accelerated toward one side, and the correspondingvelocity signal outputted on line 724 from the velocity estimator 718will show this one-sided effect, and this signal will be inputted tosummer 726. The zero force seeking loop has a low pass filter,illustrated at 728, for detecting this acceleration, and, with itspositive feedback, magnifying this effect. It then provides signalsthrough lines 730, which are amplified by power amplifiers 704, andamplified corrective signals are then sent via lines 732 to themagnetizing coils.

In order to obtain an estimate of rotor velocity for input to velocityestimator 718 in a “sensorless” manner, the back EMF across themagnetizing coil means 322 and the signal of the current flowing throughthe coil 322 may be tapped, and the rotor vibration velocity recreatedtherefrom digitally or by analog means. The rotor axial velocity signal(dX/dt) inherently exists in the back EMF (E) of the thrust bearingmagnetizing coil 322, i.e., as follows:

E=L(dI/dt)+(LI _(b) /g ₀)(dX/dt)+IR

where

L=coil inductance

I=total current flowing through coil including bias current I_(b)

R=coil resistance.

By measuring E and I in the coil 322 and re-creating two voltage signalsL(dI/dt) and IR, the velocity can be recovered using the above equation.

Since this “sensorless” method is sensitive to coil temperature whichaffects the copper wire electrical resistance, suitable search coils maybe used to pick up the EMF and eliminate the current variation part ofthe signal. Alternatively, a velocity probe using a permanent magnetmoving inside a coil may be used. Other suitable methods such as, forexample, a search coil or a Hall-effect sensor for flux measurements forobtaining rotor velocity may be used.

Since the control circuitry 710 is activated by velocity, the rotor 310at rest needs a “kick” to get the magnetic levitation started. Before“kicking” the rotor 310, it is necessary to know whether the rotor isresting or leaning on the foil or the magnetic bearing part. The “kick”should be in the direction to “free” the rotor 310. When the rotor 310is at rest, the zero force seeking loop output is to ground, illustratedat 731, and the ground switch 733 is closed. To initiate a “start,” asmall DC voltage, with the correct sign for the direction in which the“kick” is to be made, is applied through line 734 to summer 726, and thegrounding switch 733 is simultaneously opened. The zero force seekingloop 722 will then integrate this DC signal and demand a current, whichwill then be amplified by the amplifier 704, and the amplified currentapplied via the appropriate line 732 to the appropriate magnetizing coilto “shoot” the rotor 310 into the “air.” The velocity feedbackcontroller would then “grab” the rotor in the “air,” so to speak. Afterthe levitation, the DC voltage is then removed from line 734, and the“start” process is complete.

Since the bias flux is created with a permanent magnet, the thrustbearing part consumes essentially no power for maintaining the rotorsubstantially at the balance or equilibrium position. As in otherconventional bearing-rotor systems, some amount of dynamic current maystill be needed to counteract disturbances such as those due tounbalanced forces. For example, the power consumption may be less thanabout ½ watt compared to about 5 watts or more for conventional controlsystems.

Alternatively, the “smart” supervising controller 346 may include asensorless magnetic thrust part, as described above, as a back-up foruse in case of displacement sensor malfunction. See “Design and Analysisof a Sensorless Magnetic Damper”, by H. M. Chen (one of the inventors ofthe present invention), presented at ASME Turbo Expo, Jun. 5-8, 1995,Houston, Tex., 95GT180.

To make a double-acting hybrid thrust bearing, an assembly which is amirror image of the bearing 300 may be added to the rotor.

To provide load-sharing for the hybrid journal bearing 20 without“thinking,” the rotor in the foil bearing part should be at aneccentricity emax at the operating speed and under the capacity load Wf.This defines the maximum clearance circle that the rotor may traverse atthe hybrid capacity load. At the same time, the magnetic bearing partshould take its capacity load Wm along one of the two identical butindependently controlled axes (this being a conservative assumptionbecause between-axes load capacity is the square root of 2 times Wm). Tosatisfy this condition, the static deflection in the magnetic bearingpart should be

e _(max) ≈W _(m) /K _(dc)

and

K_(dc)=K_(i)G_(p)G_(a)(C_(p)+C_(i))−K_(m)

where

K_(i)=current stiffness

K_(m)=position stiffness

C_(p)=proportional gain

C_(i)=integral gain

G_(p)=displacement sensor sensitivity

G_(a)=power amplifier sensitivity.

the above equations imply that the static magnetic stiffness Kdc can notbe set too high in the hybrid bearing or the magnetic bearing part maybe flux-saturated before the foil part can share the load. Therefore,the integral gain Ci should likely be reduced to a lower value after astart-up or foil lift-off. Another magnetic relation along the loadedaxis is

W _(m) =f{(2I _(b))/(g ₀ +e _(max))}²

where

f=magnetic force constant

I_(b)=bias current with concentric air gap

g₀=concentric magnetic air gap.

The above equation implies that each quadrant of the magnetic bearingpart is provided with a bias current Ib when the rotor is placedconcentrically in the bearing. When the rotor is at its maximumeccentricity, the loaded quadrant coil has a DC current 2Ib while theopposite quadrant bias current is reduced to zero (or a small positivevalue). The amount of current 2Ib may be the design limit of the poweramplifiers or the flux saturation current of the lamination materialwith the air gap g0+emax.

FIG. 18 illustrates at 800 a representative eccentricity locus of thehybrid bearing 20 compared with the locus, illustrated at 802, of thefoil bearing part by itself. This illustrates that the hybrid locus 800is being “squashed” by the horizontal force of the magnetic bearingpart. Referring to FIG. 19, without the magnetic bearing part, the foilload W is balanced by the foil radial reactions Fr and tangential forceFt as in any fluid film bearings. With the magnetic bearing part, thehorizontal magnetic reaction will force the vector set W,Fr,Ft to rotatein the anti-rotor-rotation direction by an angle delta theta, which issmaller than the altitude angle theta itself. Therefore,

F _(mx) =−W sin (Δφ)

and

W _(i) =W cos (Δφ)+F _(my)

where

F _(mx) =f{[(I _(b) +i _(x))/(g ₀ −e _(x))]²−[(I _(b) −i _(x))/(g ₀ +e_(x))]²}

F _(my) =f{[(I _(b) +i _(y))/(g ₀ +e _(y))]²−[(I _(b) −i _(y))/(g ₀ +e_(y))]²}

e _(x) =e sin (φ−Δφ)

e _(y) =−e cos (φ−Δφ)

i _(x) =−G _(p) G ₂(C _(p) +C _(i))e _(x)

i _(y) =−G _(p) G ₂(C _(p) +C _(i))e _(y)

Referring to FIG. 5, there is shown the hybrid journal bearing 20modified to eliminate the supervisory controller. Instead of thesupervisory controller, in order that load-sharing may be achievedwithout “thinking,” an adaptive filter 21, which may be a conventionaltracking notch filter, which receives input via line 23 of rotor spinspeed, as sensed by speed sensor 25 or other suitable means, asillustrated by square wave form 27 (one square wave per rotorrevolution), and utilizes this input to continually re-set, on the flyand without “thinking,”, the filter notch at the rotor spin speed forfiltering from the input signal from the PID controller 78 to amplifiers88 a and 88 b the rotor spin speed component thereof for delivery vialine 29 to summer 31. The PID output signal is input to the adaptivefilter after its passage through the summer 31 via line 33. Thus, thefiltered signal (containing only the rotor spin speed component and, ifdesired, harmonics thereof up to, for example, the fifth harmonic) issent to summer 31 with the result that the rotor spin speed component iscanceled out, leaving components of the signal due to rotormisalignment, imperfections, and static components etc. for delivery tothe amplifiers 88 a and 88 c. By extracting the rotor spin speedcomponents of the signal, the synchronous response is forced to zero atpoint 35. By eliminating the components of the control voltage which areat the rotor spin frequency, the rotor is allowed to spin about its masscenter of gravity (commonly known as “virtual balancing”) instead of itsgeometric center thereby using less energy while reducing housingvibrations, and load sharing will occur by maintaining some softness(reduced magnetic stiffness) in the magnetic bearing part so that thecontrol current need not be re-calculated at different rotor spin speedsand, thus, so that load sharing may be achieved quickly without“thinking.”

FIGS. 20 and 21 illustrate at 800 and 900 respectively alternativeembodiments of the control systems for the hybrid foil-magnetic journaland thrust bearings respectively shown in FIGS. 2 and 15 respectively,wherein the bearing and control components are similar respectivelyexcept as shown and described herein. In order to eliminate thesupervising controller so that load-sharing may be achieved quicklywithout “thinking,” in accordance with a preferred embodiment of thepresent invention, the integral gain of the input to the PID controlleris decreased, preferably gradually, at operating speed and preferably atspeeds leading up to operating speed so as to reduce the staticstiffness thereby allowing some of the load to be assumed by the foilbearing part at these higher speeds.

Referring to FIG. 20, a suitable means, illustrated at 802, formodifying the integral gain or integral control coefficient is providedfor the input to the PID controller 78. Modification of the integralcontrol coefficient and means for doing so are within the knowledge ofthose of ordinary skill in the art to which this invention pertains. Theintegral gain modifier 802 is responsive to signals, via line 804, ofrotor spin speed from sensor 25. Thus, at predetermined rotor spin speedor speeds, such as operating speed, at which it is desired that the foilbearing part share the load, the integral control coefficient is reduced(for example, gradually as speed is increased), and, as speed isreduced, the integral control coefficient is accordingly reduced so thatthe magnetic bearing part assumes all of the load again at shut-down.The integral control coefficient is reduced in order to reduce themagnetic bearing part static stiffness so as to allow “seeking” of thefoil bearing part eccentric center at the particular speed wherebyload-sharing may be achieved quickly without “thinking.”

Referring to FIG. 21, a suitable means, illustrated at 902, formodifying the integral gain or integral control coefficient is providedfor the input to the PID controller 338. The integral gain modifier 902is responsive to signals, via line 904, of rotor spin speed from sensor331. Thus, at predetermined rotor spin speed or speeds, such asoperating speed, at which it is desired that the foil bearing part sharethe load, the integral control coefficient is reduced (for example,gradually as speed is increased), and, as speed is reduced, the integralcontrol coefficient is accordingly increased so that the magneticbearing part assumes all of the load again at shut-down. The integralcontrol coefficient is reduced in order that, regardless of rotor speed,the control current will not need to be re-calculated at different rotorspeeds and, thus, so that load-sharing may be achieved quickly without“thinking.”

It should be understood that, while the present invention has beendescribed in detail herein, the invention can be embodied otherwisewithout departing from the principles thereof, and such otherembodiments are meant to come within the scope of the present inventionas defined by the appended claims.

What is claimed is:
 1. A method for bearing a rotor portion comprisingsharing rotor portion load between a foil bearing part and a magneticbearing part without using a calculated rotor trajectory correspondingto various rotor spin speeds and further comprising controlling powerinput to the magnetic bearing part in response to feedback of rotorposition, wherein the step of sharing load comprises modifying the powerinput in response to rotor spin speed to reduce load capacity of themagnetic bearing part as rotor spin speed is increased and increase loadcapacity of the magnetic bearing part as rotor spin speed is decreased.2. A method according to claim 1 wherein the step of sharing loadcomprises outputting, in response to input of rotor portion radialposition from a first sensor on a first side of a housing for the foiland magnetic bearing parts, a signal to a first control coil forregulating amount of flux on the first side of the housing and furthercomprises outputting, in response to input of rotor portion radialposition from a second sensor on a second side of the housing, a signalto a second control coil for regulating amount of flux on the secondside of the housing.
 3. A method for bearing a rotor portion comprisingsharing rotor portion load between a foil bearing part and a magneticbearing part without using a calculated rotor trajectory correspondingto various rotor spin speeds and further comprising controlling powerinput to the magnetic bearing part in response to feedback of rotorposition, wherein the step of sharing load comprises modifying the powerinput in response to rotor spin speed to reduce load capacity of themagnetic bearing part as rotor spin speed is increased and increase loadcapacity of the magnetic bearing part as rotor spin speed is decreased,wherein the step of modifying power input comprises reducing integralgain of input to a controller for controlling the power input as therotor spin speed is increased and increasing integral gain of the inputto the controller as the rotor spin speed is decreased.
 4. A method forbearing thrust of a rotor comprising disposing foil and magnetic bearingparts on opposite sides axially respectively of a portion of the rotorfor bearing thrust, controlling power input to the magnetic bearing partin response to feedback of rotor axial position, and sharing on the flyrotor portion thrust load between the foil bearing part and the magneticbearing part, wherein the step of sharing thrust load comprisesmodifying the power input in response to rotor spin speed to reduce loadcapacity of the magnetic bearing part as rotor spin speed is increasedand increase load capacity of the magnetic bearing part as rotor spinspeed is decreased.
 5. A method according to claim 4 wherein the step ofmodifying power input comprises reducing integral gain of input to acontroller for controlling the power input as the rotor spin speed isincreased and increasing integral gain of the input to the controller asthe rotor spin speed is decreased.
 6. A method according to claim 5further comprising continually deriving a signal of rotor portionvelocity from back EMF or flux of a magnetizing coil of the magneticbearing part, and passing the signal through a zero force seeking loopto provide signals to the magnetizing coil for continuously returningthe rotor portion to an equilibrium position.
 7. A method for bearing arotor portion comprising sharing rotor portion load between a foilbearing part and a magnetic bearing part without using a calculatedrotor trajectory corresponding to various rotor spin speeds and furthercomprising controlling power input to the magnetic bearing part inresponse to feedback of rotor position, wherein the step of sharing loadcomprises modifying the power input in response to rotor spin speed toreduce load capacity of the magnetic bearing part as rotor spin speed isincreased and increase load capacity of the magnetic bearing part asrotor spin speed is decreased, wherein the step of modifying power inputincludes inputting rotor spin speed to an adaptive filter to continuallyreset the adaptive filter at the rotor spin speed as the rotor spinspeed is changed.